How to Use a Speed Square for Angles and Basic Layout

What the parts of a speed square do

A speed square is a rigid right triangle with a lipped edge on one side. That lip, called the heel or fence, hooks over the board so the tool stays put while you mark. Most squares also carry a degree scale, and some carry rafter scales for roof work.

The corner where the heel meets the long edge is the pivot point. Think of it as a hinge. Everything you do with angles comes from holding that corner on the board edge and swinging the square around it.

Not every square has every feature. On our best speed square page, the 7 in Swanson S0101 lists a pivot point, rafter tables, and scribe notches. The 8 in Swanson Speedlite lists scribe notches but no pivot point, so it suits simple square and 45° marks better than angle work.

• Heel (fence): hooks the board edge so the square sits square to it.
• Pivot point: the corner you rotate around to set an angle.
• Degree scale: lets you read and mark angles from the pivot.
• Rafter scales: used for common, hip, and valley roof cuts.
• Scribe notches: slots that hold a pencil for parallel lines.

Marking 90° and 45° lines

These are the two marks you will make most often. Start with a flat, straight board edge, because the heel only works as a reference if the edge is clean. Brush off sawdust first.

For a 90° line, push the heel firmly against the board edge. Slide the square to your measurement and draw along the edge that runs across the board. Hold the pencil at the same slight angle for the whole line so the mark stays thin and honest.

For a 45° line, keep the heel against the edge and use the angled side (the hypotenuse) instead. Draw along it for a miter on trim or a corner brace. Flip the square over to mark the opposite 45° direction.

• Press the heel tight against the board edge.
• Slide to your measurement mark.
• Draw one steady line with a sharp pencil.
• Put an X on the waste side so you cut the right piece.

Setting any angle with the pivot

When you need an angle other than 90° or 45°, use the pivot method. Place the pivot point on the edge of the board, right at the spot where your line should start. Keep that corner in place.

Now rotate the square until the degree you want lines up with the board edge. Read the number where the scale meets the edge, not where it meets your hand. Once it lines up, hold the square still and draw along the straight edge facing your line.

Squares with a full scale, like the 0° to 90° etched scale listed on the Swanson 2-piece kit or the preset angle guide on the Klein kit, make this easier to read. If a square only lists 45° and 90° slots, treat those as your reliable marks and use a separate tool for other angles.

Basic layout: parallel lines, saw guide, and rafters

Scribe notches give you a quick way to draw a line parallel to an edge. Set the heel against the edge, put the pencil in the notch you want, and slide the square along the board. The line follows the edge exactly. The Swanson S0101 lists a 1/4 in scribe bar, while the Klein kit lists 1/8 in notches on the 4-1/2 in square and 1/4 in notches on the 7 in square.

The heel also works as a saw guide. Mark your line, set the square so the heel hooks the board, and slide the saw's base plate against the square's edge. That keeps a circular saw cut square without a separate fence. Make sure the square cannot shift, and keep your hand clear of the blade path.

For roof rafters, the common scale turns a roof pitch into a mark. For a 6 in 12 pitch, set the pivot on the board edge, rotate until 6 on the common scale meets that edge, and mark along the straight side. A guide book helps here, and our page notes that the Swanson S0101, the Swanson 2-piece kit, and the Spec Ops square each include one.

Which features matter for which job

You do not need every feature on every job. This table matches common tasks to the features that help, using specs from our comparison page.

If you mostly square and mark 90° and 45° lines, nearly any metal square will do. If you cut roofs, look for a pivot point and rafter scales.

Check your square and work safely

Test accuracy before you trust a square. Draw a line against a straight board edge, flip the square over, and draw a second line from the same point. If the two lines match, the square is true. If they split apart, the tool is off and should not be used for precise work.

A dropped square can bend, so run this check again after any hard fall. Matte finishes, like the one listed on the Swanson S0101, cut glare when you read marks outdoors.

• Wear safety glasses whenever you cut, and hearing protection around a saw.
• Unplug a corded saw before changing blades or adjusting depth.
• Keep both hands clear of the blade path when using the square as a saw guide.
• Never rely on a square alone for roof work. Use proper fall protection, and do not climb in wet or windy conditions.
• Handle the corners carefully, since aluminum edges can be sharp.

Frequently asked questions

Do I need a speed square or a combination square?

A speed square is faster for framing, roofing, and quick 90° and 45° marks on rough lumber. A combination square has a sliding head and ruler, which suits fine measuring and setting depths. Many carpenters keep both.

Can I use a speed square on plywood or sheet goods?

Yes, as long as the edge you press the heel against is straight and clean. For wide panels, the square is too short to check the whole sheet, so use a longer straightedge for long cuts.

Why does my angle mark look slightly off?

Common causes are a dirty or damaged board edge, a pivot point that slipped, or a thick pencil line. Re-seat the square, use a sharp pencil, and cut on the waste side of the line.

Is a plastic speed square good enough?

Plastic is light and fine for simple square and 45° marks, but it is a poor match for hard framing work. Our buying guide favors aluminum alloy for daily jobsite use.